Synchronous electric motors having permanent-magnet rotors have existed for some time. Many of the rotors that have been used in such electric motors have magnets that are mounted at the periphery of the rotor surface. In these motors, the rotor typically is made of a magnetically conductive material such as iron or the like. The magnetic flux available to produce force in connection with the magnetic fields in the stator is proportional to the surface area of the magnets on the outer surface of the rotor. In these motors, great care must be taken to mount the magnets in precise relation to the axis of the rotor and so as to maintain a smooth outer surface.
In operation, the flux lines from the magnets in these motors link across an air gap to the stator. The magnets are arranged so that adjoining rows of magnets have opposite magnetic poles facing outward. Thus, around the outside circumference of the rotor, the rows of magnets are arrayed north to south to north, and so on. Typically, the rows are also slightly skewed relative to the stator or the stator is slightly skewed relative to the rotor so as to minimize cogging that occurs as the magnets line up with the respective teeth of the stator. Since total magnetic flux for a magnet is proportional to its surface area, the total available torque for these types of motors is directly linked to the total available surface area of the outside of the rotor. Thus, this rotor arrangement is most useful where either the size of the motor (size of the diameter of the rotor) does not need to be small or the total available torque does not need to be large.
There are some motors where the permanent magnets are not mounted at the outside periphery of the rotor. An example of such a motor is shown in U.S. Pat. No. 4,697,114, issued Sep. 29, 1987, to Amemiya et al. In these motors, the permanent magnets are secured between magnetically conductive wedges which are secured in fixed relation to the shaft of the rotor. The wedges in these motors sometimes consist of sets of laminated plates held in place by fastening bolts that extend through them parallel to the axis of rotation of the motor and attach to steel end plates which are securely fitted to the shaft. In these motors, the inner surfaces of the wedges and permanent magnets are radially spaced from the shaft the entire length of the magnetized rotor.
In the aforementioned motors the diameter of the rotor must be sufficiently large to accommodate the air gap between the shaft and the magnets and wedges. This presents apparent problems in a lower available torque for a given diameter rotor and a larger overall size. In addition, the manner of mounting and the positioning of the magnets and wedges would appear to adversely affect the response time of the motor to rapid changes in the signal input (stiffness) along with providing relatively high inertia, eddy currents and diminished rotor efficiency.
In control systems, it is often desirable to use small high torque devices to operate various mechanical systems. In the past, where high torque was required but space was limited, system designers often opted to use hydraulic systems because of the lack of electric motors with sufficiently high torque to size ratios. As a result, there has been a need for an electric motor with high torque that can be used in relatively small spaces.
In applications such as robotics and the like, where response time is critical, there is additional need for electric motors that have high stiffness while not requiring a significant amount of space. In addition, such applications often require that the control system maintain a high energy efficiency.
Accordingly, one object of this invention is to provide a small electric motor including a permanent-magnet rotor having embedded magnets which has high torque to size ratio.
A related object of the invention is to provide an electric motor including a permanent-magnet rotor having embedded magnets which has high stiffness while requiring a minimum amount of space.
Another related object of the invention is to provide an electric motor including a permanent-magnet rotor having embedded magnets which achieves high torque and stiffness while maintaining high efficiency.
Another related object of the invention is to provide an electric motor including a permanent-magnet rotor having embedded magnets which achieves smoothness of operation at low speeds.